A few years ago I was really into modding out my laptops -replacing CPU's, swapping GPU's, replacing motherboards so that I got the newer revision, etc...
What I can tell you from this experience is that what you wan to do is try to find an example of your laptop on the web equipped with the CPU you want to swap. Unless Dell specifically released a N5030 with a quad core, it won't work. There's a lot of other things to consider, like CPU TDP, chipset, and BIOS limitations that can get in the way of a clean swap.
With the money you'd spend on a used QC on eBay, I'd say you're better off buying a used laptop with a first or second generation i-series CPU. Laptops equipped with those are no longer that expensive on the used market, and you'll get WAY better performance versus upgrading that N5030.
Intel DZ77GA-70K - i7-3770 3.4GHz - 16GB DDR3 1866 HyperX Predator - 2x SLI eVGA GTX 780 SC 3GB - 240GB Mushkin Chronos SSD -
850 Watt Seasonic M12ii - H60 Closed Loop Cooler - Corsair Air 540 - Win 8.1 Pro x64 - QNix QX2710 27" 1440p @ 85 Hz (Samsung PLS)
1.8 GHz Intel Core i7 4500U - 8GB DDR3 1600 (2 x 4GB) - 1TB + 256GB SSD - 15.6" LED - nVidia 745m 2GB - Windows 8.1 Pro x64